the metals lanthanum, cerium, praseodymium, neodymium, promethium, and samarium, forming a sub-group of the lanthanides
cerium metals in American English
a series of closely related metals belonging to the rare-earth elements and having atomic numbers from 57 to 63; lanthanum, cerium, praseodymium, neodymium, promethium, samarium,and europium